The present invention relates to a workpiece holder for polishing which is used for precision polishing of surface of workpiece such as a semiconductor wafer, an apparatus for polishing a workpiece, and a method for polishing a workpiece.
In the conventional polishing operation for workpieces, a plate composed of a rigid material such as glass, metal and ceramics is used as a workpiece holder, and a workpiece is held by adhering it on the surface of the holder with an adhesive such as wax, or by vacuum suction using a workpiece holder surface composed of a gas-permeable porous material or a workpiece holder surface provided with multiple perforated holes for suction. The workpiece is pressed to a polishing pad adhered to a turn table while a polishing agent is poured onto the polishing pad, and the workpiece and the polishing pad are rotated to perform the polishing. As shown in FIG. 3, for holding a workpiece W on a workpiece holder 1 by vacuum suction, there is provided a structure that a workpiece holder back plate 4 having grooves 5 for vacuum is provided on a back face of a workpiece holder body 2 to attain sealing with the workpiece holder body 2 and to secure a vacuum way, so that each perforated hole 3 for suction should be communicated to the vacuum way 7 via the grooves 5 of the holder back plate 4, and thereby the workpiece W is held on a workpiece holding surface 8 by vacuum suction.
As the material of the holder back plate 4, metal or hard synthetic resin is conventionally used. In addition, to enhance sealing between the back face of workpiece holder body 2 and the holder back plate 4, an O-ring 6 or the like is also used.
However, during the polishing of the workpiece, polishing agent slurry may be sucked through a small gap between the workpiece held by vacuum suction and the workpiece holding surface of the workpiece holder body. The sucked polishing agent slurry is evaporated to dryness in a space between the back face of the holder body and the holder back plate. As a result, the polishing agent solidified in a space between the holder back plate and the back face of the holder body will be pressurized with the holder back plate by pressure generated by the vacuum suction, and slightly deform the holder body, and such deformation will be transferred to the workpiece surface being polished. Thus, the quality of the polished workpiece surface may be adversely affected. In addition, even the shapes of the grooves on the holder back plate may also be transferred to the workpiece being polished via the holder body.
Furthermore, the O-ring, which is used in order to enhance the sealing between the back face of the holder body and the holder back plate, is likely to suffer from unbalanced load. This may cause uneven distribution of stock removal for polishing within the surface, which may adversely affect flatness of the workpiece.
Therefore, the present invention was accomplished in view of the aforementioned problems, and its main object is to improve the material of holder back plate of a workpiece holder for polishing that holds a workpiece by vacuum suction to enhance sealing with the holder body, thereby developing such a holder back plate that should not transfer deformation of the holder body to the workpiece surface, even if polishing agent slurry is introduced and solidified, to provide a workpiece holder for polishing having a workpiece holding surface of high precision, an apparatus for polishing a workpiece and a method for polishing a workpiece utilizing them.
In order to achieve the aforementioned object, the present invention provides a workpiece holder for polishing comprising a workpiece holder body which is provided with multiple perforated holes for holding a workpiece by vacuum suction and a holder back plate which is closely contacted with a back face of the holder body and has grooves for vacuum, wherein the holder back plate is composed of a synthetic resin and has an Asker C hardness of 70 or higher but lower than 98.
During the polishing of a workpiece, polishing agent slurry may be sucked through a small gap between the workpiece held by vacuum suction and the workpiece holding surface of the workpiece holder body. The sucked polishing agent slurry is solidified by evaporation in a space between the back face of the holder body and the holder back plate. As a result, patterns of the solidified polishing agent will be pressurized with the holder back plate by pressure generated by the vacuum suction. However, if a synthetic resin having an Asker C hardness of 70 or higher but lower than 98 is used as the material of the holder back plate as defined above, the holder back plate should be constituted with a soft resin, and therefore the pressure is absorbed by it. Accordingly, the holder body is no longer deformed, and thus deformation is no longer transferred to the workpiece surface during polishing. Furthermore, the shapes of the grooves themselves for vacuum on the holder back plate are also no longer transferred to the workpiece being polished via the holder body. Therefore, by using a holder back plate composed of a resin having a hardness defined by the present invention, highly precise workpiece polishing can be realized with desired high degree of flatness without causing waviness.
Furthermore, since it can have a moderate hardness as a holder back plate, it shows superior adhesion with the back face of the holder body, and leak of the outer air through this adhered surface hardly occurs. Therefore, it becomes unnecessary to use an O-ring, which is conventionally used for enhancing the adhesion, and uneven distribution of stock removal of the surface during polishing the workpiece caused by unbalanced load applied on the O-ring, and bad influence on the flatness of the workpiece are substantially eliminated. Thus, highly precise polishing can be attained to such an extent that waviness is not observed on the workpiece surface even with a magic mirror.
In the aforementioned workpiece holder, the synthetic resin can be one selected from urethane resins, vinyl chloride resins and polyamide resins.
If the material of the synthetic resin is selected from those mentioned above, the range of Asker C hardness required for the present invention can be well satisfied.
The present invention also provides an apparatus for polishing a workpiece comprising a turn table adhered with a polishing pad, means for feeding a polishing agent on a surface of the polishing pad and a workpiece holder for polishing for forcibly pressing a workpiece to the surface of the polishing pad, wherein the workpiece holder for polishing is the one defined above.
By using an apparatus for polishing provided with a workpiece holder for polishing which comprises of a holder back plate made of a resin having a hardness defined by the present invention and a workpiece holder body as described above, a workpiece can be finished by polishing as a workpiece of high precision with desired high degree of flatness and no waviness on the workpiece surface even when observed by a magic mirror. In particular, when the workpiece is a semiconductor wafer, yield and productivity of highly integrated devices in a highly integrated device production step can be improved.
Further, the present invention also provides a method for polishing a workpiece, which comprises holding a back face of a workpiece by vacuum suction on the surface of the aforementioned workpiece holder for polishing used as a workpiece holding surface, and then polishing a surface of the workpiece by bringing the workpiece into contact with a polishing pad.
According to the method of the present invention, because of the use of a synthetic resin having a hardness within a specific range as the material of the holder back plate, moderate cushioning property can be imparted to the holder body. Therefore, the deformation of the holder body, which is caused by the polishing agent that penetrates through a gap between the workpiece and the holder body and solidifies in a space between the back face of the holder body and the holder back plate, is absorbed, and the transfer of this deformation to the workpiece can be prevented. In addition, the transfer of the shapes of grooves themselves for vacuum of the holder back plate to the workpiece via the holder body is also prevented. Therefore, according to the method of the present invention, there can be realized highly precise workpiece polishing with desired high degree of flatness without causing waviness on the workpiece surface even when observed by a magic mirror.
As explained above, according to the present invention, a workpiece having excellent flatness and no waviness on the workpiece surface can be stably produced, thanks to the adsorption of the synthetic resin having an Asker C hardness within the specific range as the material of the holder back plate that constitutes the workpiece holder of a polishing head portion. Although an O-ring is provided in conventional holder back plates for enhancing sealing between the back face of the holder body and the holder back plate, it becomes unnecessary in the present invention because of the superior sealing property of the holder back plate itself. This eliminates possibility that uneven distribution of stock removal of the surface is generated by unbalanced load applied on the O-ring or the flatness of the workpiece is greatly affected.